This invention relates generally to fuses and more particularly to fuses used in semiconductor integrated circuits.
As is known in the art, many modern semiconductor integrated circuits include fuses to protect sensitive parts during the manufacturing process as well as for the activation of redundant circuits, such as redundant memory cells in the case of Dynamic Random Access Memories (DRAMs). There are typically two types of fuses; a laser-blowable fuse, and an electrically (e.g. current) blowable-fuse. Electrically blowable fuses provide advantage over laser-blowable fuses in terms of size.
One technique used in the fabrication of an electrically blowable fuse is to cover the fuse material with surrounding dielectric material, such as silicon dioxide or BPSG material. After the fuse material has blown however, over time the material may migrate (i.e., heal) and provide an unwanted short circuit condition. Further, when the fuse is blown, mechanical forces in the surrounding dielectric are produced which may generate cracks in the dielectric material as it expands from the explosion of the fuse material. These explosion effects may damage other neighboring fuses.
In another technique, a cavity is formed over the fuse. Thus, when the fuse is blown to provide a open circuit, the fuse material becomes somewhat contained within the provided cavity. With DRAMs, these fuses are typically doped polycrystalline silicon having an upper layer of tungsten silicide. Further, these fuses are typically formed with the formation of the gate electrodes of the DRAM cells. While the gate electrodes are formed over active regions in the semiconductor, the fuses are typically formed over silicon dioxide isolation regions used to electrically isolate the active regions. The cavity is sometimes formed by a specific photolithographic step which opens an aperture in a mask over the fuse area while the remainder of the chip (i.e., the active regions) is protected from the series of dry and wet etch steps used to form the cavity. More particularly, the cavity is typically formed selectively between the fuse material and an surrounding insulator, typically silicon nitride. Thus, the typical gate structure (or gate stack) and fuse both include a conductor made up of doped polycrystalline silicon/tungsten silicide encapsulated in a silicon nitride insulator which is selective removed over the fuse to form a cavity for the fuse blown material. This cavity is typically sealed with a plasma deposited silicon dioxide leaving a pocket, i.e.e, the cavity described above, for the blown fuse material. In any event, this later technique requires a separate masking step in the fabrication process.